Method and apparatus for control of layer thicknesses

ABSTRACT

It is shown a method and apparatus for distributing a viscous liquid over a surface of a substrate, e.g. a semiconductor wafer or a datastorage media, by conditioning the substrate thermally, locally specific before or during the spin coating process.

CROSS REFERENCE TO RELATED APPLICATON

[0001] This application claims priority of U.S. Appl. No. 60/431,346,which is incorporated herein by reference.

FIELD OF THE INVENTION

[0002] This invention generally relates to the field of spin coating ofsubstrates, especially to a method and apparatus for controlling thethickness distribution of the coating by controlling the temperaturedistribution of the coating on the substrate.

BACKGROUND OF THE INVENTION

[0003] As is well known in the prior art, especially in the field ofsemiconductor manufacturing but also in certain areas of optics orbiotechnology, a homogenous distribution of liquids on an essentiallyplane substrate may be achieved by rotating (spinning)a substrate aroundan axis normal to the plane given by its surface. By applying a viscousliquid onto the surface during spinning centrifugal forces affect adistribution of the liquid radially outwards over the surface. Such“spinning” technique is used to disperse e.g. lacquer, resins, photoresist on semiconductor substrates. Moreover it is utilized in theproduction of optical data storage technology to provide an essentiallyhomogeneous layer of resin, lacquer, adhesive etc. A special case is theproduction of all type of DVD formats which require the bonding of twohalf-disks.

[0004] A standard process for such distribution method is:

[0005] 1) Dispensing a liquid on the substrate to be coated; eventuallyrotating it slowly during this step to achieve a advantageous initialspreading.

[0006] 2) Spinning the disk at high speed (typically a few hundred rpmup to 12.000 rpm) to homogeneously distribute the liquid. The thicknessof the layer depends on parameters such as viscosity, temperature,rotation speed and rotation time.

[0007] For substrates with a center hole the profile of the spin coatedlayer thickness shows a low-high trend from the inner radius towards theouter edge. This is due to the fact that there is no liquid materialat/close to the center hole which could flow outwards. This lack ofmaterial causes the reduced thickness at small radii.

[0008] The variation of the thickness distribution therefore will not bereduced to a minimized level by standard spin coating process. In orderto achieve an optimized coating condition, an extra treatment during thespin coating process is required.

[0009] Moreover, for substrates without a center hole it is difficult toachieve ramped thickness distributions, as would be of interest e.g. forphoto resist coating thickness in mastering applications.

[0010] It is therefore desirable to have a method to influence theradial thickness distribution during the spinning process. The radialthickness dependence of a liquids thickness is dictated by the physicsof the spinning process and cannot be avoided with radially constantviscosity of the liquid. The objective of the invention therefore is toprovide a method for controlling the viscosity of the liquid to bedistributed during spinning.

DESCRIPTION OF PRIOR ART

[0011] In semiconductor manufacturing the use of cooling or heatingchucks is widely known. Onto these chucks the semiconductor substrates,normally a hafer, are clamped and by thermal contact with the(heated orcooled) chuck the substrates temperature is adjusted according to theneeds of the particular processing step. However, such chucks seek toadjust temperatures homogenously over the complete area of thesubstrate.

[0012] A use and continuation of that principle is being shown in U.S.Pat. No. 6,242,044 B1. This document describes a method to adjust thetemperature of a CD radially from the center to the edge. This isachieved in an embodiment with a central rotating shaft with a thereonmounted plate for carrying the CD.

[0013] The bottom of the shaft only is being actively cooled and viathermal conductivity a thermal gradient over the plate is being formed.The drawbacks of this method are as follows: The temperature gradient isbeing dependant on the material of the chuck and the environmentalconditions such as ambient temperature. Moreover the system will needtime to achieve a stable condition, i.e. the thermal gradient mustdevelop. Furthermore polycarbonate is a relatively good thermalinsulator, therefore cooling and/or heating a liquid dispensed on thesurface via the plastic substrate is inefficient.

SUMMARY OF THE INVENTION

[0014] To influence the liquids viscosity in a spin coating process,especially the resin layer thickness distribution of spin-coatedsubstrates with a center hole (e.g. optical disks like DVD, CD, Blu-Ray. . . etc.), a temperature gradient is created locally selectivelybefore or during a spinning process by a heat source directed to theside of the substrate where the liquid is distributed on. Alternativelya cooling source can be used to achieve such a temperature gradient.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0015] One embodiment of the invention comprises a stream of hot(temperature range 40-90° C.) gas directed on the outer radius of thedisk during spinning, in order to change the liquids viscosity over theradius of the disk.

[0016] In another embodiment, the stream of hot gas is directed viasuitable means onto several areas of the substrate. Another embodimentallows to control temperature and/or flow of gas selectively toprecisely control the liquids viscosity and thereby thicknessdistribution of the liquid on the spin coated substrate. “Gas” is to beunderstood in the broadest manner, such as simple air or cleaned gasesprovided from gas reservoirs. Heating of the gas may be achievedelectrically via a resistance heating or other technical suitable means.

[0017] This thermal conditioning may in another embodiment of theinvention be achieved by sources of electromagnetic radiation. This maycomprise lamps with essential visible spectra or IR spectra.

[0018] In a further emodiment the inventions thermal source may compriseseveral sub sources, which can advantageously be directed to differentlocations of the substrate during spinning. This results in a control ofthermal conditions in an essential circular region around the axis ofrotation. Technically this may be achieved by e.g. a plurality ofnozzles, directing heated or cooled gases to such regions of thesubstrate. Another variety is to use a plurality of lamps, such as IRradiators or halogen lamps, being arranged to aim at different locationsof the substrate.

[0019] In case of gas nozzles, thermal conditioning of the substrate caneasily be adjusted by regulating gas flow and/or temperature of the gas.In case of lamps this can be accomplished by regulating the powerdistributed to the respective lamp. For both cases the angle of thenozzle or lamp with respect to the plane of the substrate is a furtherparameter to finetune the thermal conditioning.

[0020] An apparatus suitable to implement the invention may comprise arotatable support, dispensing means to spread a liquid on the surface ofthe substrate and means to fasten at least one thermal source in aposition with respect to the substrate, where it can influence thethermal condition of the substrate. In one embodiment this fasteningmeans may comprise a cover with the thermal source and, eventually,dispensing means affixed to it.

[0021] In a preferred embodiment the thermal sources are attached to anarm, which extends over at least a portion of the rotatable support.This arm, cover or in general fastening means may be movable in a way toclear the area of the substrate/support in order to load and unload thesubstrate from and to the support.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a cross sectional view of a rotational support with asubstrate.

[0023]FIG. 2 shows an embodiment of the invention with an arm extendingover the substrate with several thermal sources.

[0024]FIG. 3 shows the result of an experiment according to theinvention

DETAILED DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 shows a substrate 1 on a support 2. The substrate has acenter hole which allows to centering it around a rotation axis 3. Thearrow indicates the rotation of support and substrate. Driving means forrotation are not shown. During rotation excess liquid is being spinnedoutwards, shields 4 and 6 protect the environment from such excessliquid.

[0026]FIG. 2 shows the embodiment of FIG. 1 with a movable arm 11 with asource of radiation 10. The movability of arm 11 is indicated by arrows12. As can easily be understood, the arm may be moved linearity, pivotedhorizontally or vertically or in any other suitable manner to clear thearea of substrate and support. This may be especially necessary when thesubstrate is being loaded or unloaded from the support. Loading andunloading mechanisms are not shown, they can be construed according tothe state of the art.

[0027] Arrows 13 indicate the effective direction of a plurality ofthermal sub sources.

[0028] In an application of the inventive method in an apparatusaccording to the invention, substrate 1 is being placed by suitablemeans on support 2. Substrate 1 may be a semiconductor wafer, adatastorage media such as a CD, DVD or alike or any essentially flatworkpiece. In a preferred embodiment arm 11 carries as well thermalsources as dispensing means, so after placing the arm above thesubstrate liquid could be dispensed. This way the liquid is being spreadupon the surface of substrate 1, eventually while being rotated slowly.After spreading the liquid the spinning of the substrate is initiatedand in parallel to or shortly after start of spinning the thermalconditioning begins. The conditioning time can be defined by precedentexperiments, or alternatively contact free measurement means determinethe end of the thermal conditioning.

[0029] Furtheron edge effects, like the build up of droplets at the edgeof spin coated substrates can be avoided by selectively directing athermal source at such edge.

[0030] Basically the thermal source(s) locally influence the viscosityof the liquid, such that a desired distribution, homogenous or aselectively inhomogenous, on the surface of the substrate is beingachieved.

[0031] In another way of using the invention the order of steps may bechanged, in a way, that the substrate is being preheated in the waydescribed, the liquid being dispensed and spinned afterwards. This maybe advantageous for applications, where e.g. mechanical, physical orchemical constraints do not allow the order described beforehand. Inthis case the thermal capacity of the substrate must be sufficient tohold the thermal gradient until the liquids visosity can be influenced.

[0032] The inventive method may be performed at a single processstation, where the thermal source, dispensing means and rotatable meansare being joined. However it may be beneficial to distribute the stepsof thermal conditioning, dispensing and spinning to several processmodules.

[0033]FIG. 3 shows results of a single heat source (flow of heated air)being directed to an outer region of an optical storage media duringspin coating with resin. Comparison to an unconditioned sample showssignificant improvement of layer homogeneity. In contrast to not heatedsubstrates with monotone increasing thickness as a function of theradius, the thickness of the partially heated substrates decreases afterreaching a maximum at a radius of about 35-45 mm.

[0034] The thermal gradient applied between different areas of asubstrate was just a few degrees, in case of a bonding lacquer on a DVDhalf disk about 4° C. Depending on viscosity and other environmentalconditions a thermal gradient of about 10

C is sufficient for most applications.

What is claimed is:
 1. A method for distributing a viscous liquid over asurface of a substrate, comprising the steps: placing a substrateessentially horizontal on a support applying a viscous liquid onto asurface of said substrate rotating the substrate to distribute theliquid radially outwards and conditioning the liquid on the substratethermally, to influence its viscosity locally in a specific way.
 2. Amethod according to claim 1, wherein the thermal conditioning is beingeffected by a thermal source of heat or cold placed above the surface ofthe substrate
 3. A method according to claim 1, wherein the thermalconditioning is being effected by a stream of heated or cooled gas.
 4. Amethod according to claim 1, wherein the thermal conditioning is beingeffected by a source of electromagnetic radiation.
 5. A method accordingto claim 4, wherein the source of radiation is a lamp with essentiallyvisible spectra or an IR radiator.
 6. A method according to claim 2,wherein the thermal source comprises at least two sub sources.
 7. Amethod according to claim 6, wherein the sub-sources are directed todifferent positions with regard to the radius on the substrate. 8.Apparatus for thermal conditioning a liquid on a rotating substratecomprising: a rotatable support, dispensing means for a liquid to bedistributed on the substrate surface and fastening means for at leastone thermal source placed above the substrate.
 9. Apparatus according toclaim 8, wherein the fastening means comprise a cover, extending over atleast a part of the support.
 10. Apparatus according to claim 8, whereinthe fastening means is construed as an arm, extending over at least aportion of the support.
 11. Apparatus according to claim 8, wherein thethermal source comprises one of at least a source of radiation, a lamp,an IR radiator, a stream of hot or cool gas.
 12. Apparatus according toclaim 8, wherein the dispensing means are mechanically affixed to thefastening means.
 13. Apparatus according to claim 8, wherein thefastening means are movable with respect to substrate and support inorder to remove the arm at least during loading and unloading of thesubstrate.
 14. A method for distributing a viscous liquid over a surfaceof a substrate, comprising the steps: rotating the substrate on anessentially horizontal support and conditioning the substrate thermally,locally specific applying a liquid onto a surface of said substraterotating the substrate such that the liquid is being radiallydistributed according to a desired distribution.